Method of manufacturing evacuated devices



p 1970 s. R. SMITH, JR 3,531,175

METHOD OF MANUFACTURING EVACUATED DEVICES Filed Feb. 19, 1969 United States Patent 3,531,175 METHOD OF MANUFACTURING EVACUATED DEVICES Sidney R. Smith, Jr., Mrytle Beach, S.C., assignor to General Electric Company, a corporation of New York Filed Feb. 19, 1968, Ser. No. 706,295 Int. Cl. H01j 17/26 US. Cl. 316-25 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates generally to evacuated devices and pertains more particularly to new and improved methods for manufacturing such devices.

Because a wide variety of devices in many fields of manufacture incorporate vacuum chambers or evacuated envelopes of one form or another, many different methods for evacuating such envelopes have been developed in the prior art. The selection of any one of these methods for manufacturing a given device is usually determined by; the degree or hardness of the vacuum that must be formed, the nature and, volume of the device being formed, and the relative cost of the respective processes. While the nature of the device ordinarily establishes rigid, unchangeable parameters, the type of processes to which it is amenable may frequently be varied to advantage, and of course any resultant reduction in manufacturing costs realized from such a change would usually strongly recommend its adoption. The various methods for forming a vacuum chamber known in the prior art can be considered in several broad categories. For example, some methods are readily adaptable for use in batch manufacturing processes, whereas other methods find primary utility in continuous flow, or assembly line type, processes. Another subdivision of prior art evacuating method can be made between various pumping processes for developing a vacuum chamber and the pumpless processes that relay primarily on getter materials in combination with reducing atmospheres and controlled temperature cycles to form vacuums in such chambers.

Several basic problems are inherent in each of these prior art methods. For example, when it is necessary to form a hard vacuum with any of the diffusion pump or ion pump processes, the type of equipment required to effect the evacuating operation is expensive, and the process itself takes a substantial amount of time to develop the desired degree of evacuation, thus, further increasing manufacturing expense. Whereas the various pumpless processes generally represent a substantial improvement in the area of manufacturing cost reduction, since they can be employed in a continuous-flow operation that forms a hard vacuum relatively rapidly, they ice also involve several serious drawbacks. Perhaps the most important of these drawbacks stems from the fact that highly active gases such as hydrogen, are normally used to reduce the oxides on the surfaces of the evacuated device prior to sealing it. These active gases frequently create a hazardously exposive working atmosphere which is often rendered more dangerous due to the fact that the evacuating process normally requires the employment of severe temperature cycles, which in turn demand, the use of heating equipment that may ignite the explosive atmosphere. A second inherent drawback of pumpless evacuating processes is attributable to the fact that the commercially available reducing gases utilized in such processes normally contain a small but significant percentage of impurities, such as argon, which the commonly used gettering materials cannot sorb. Therefore, the vacuum chamber formed with such processes is defective in that these impurities remain in the chamher. In applications such as vacuum gaps or evacuated electric discharge devices or electric switches, the existence of such impurities in the vacuum chamber can adversely affect their ability to operate properly as well as shorten the operating life of the component parts. Another shortcoming of the pumpless evacuating processes known in the prior art stems from the fact that a large amount of temperature responsive gas removing material, such as titanium, must be mounted within the evacuated chamber to afford adequately complete sorption of the very substantial amounts of reducing gas atmosphere remaining in the chamber at the time it is sealed. Since such materials are relatively expensive, it is desirable to reduce the amount necessary to form each evacuated device, because once it is sealed within the device it is not retrievable for subsequent re-use.

The manufacturing process of my invention overcomes the above mentioned problems inherent in known prior art methods. Thus, it affords an easily facilitated, reliable and economical process whereby chambers or envelopes may be exhausted to the required degree of evacuation within a very limited amount of time.

Accordingly, one object of my invention is to provide an inexpensive method for manufacturing high quality evacuated chambers or envelopes which eliminates the need to employ complicated or expensive pumping apparatus.

Another object of my invention is to provide a new and improved method of manufacturing electric discharge devices which obviates the need for supplying active reducing gas to the enclosure to be evacuated prior to the time it is sealed.

A further object of the invention is to provide a method for manufacturing evacuated devices having a gas sorbing and desorbing material therein, wherein the amount of such material necessary for effectively forming a hard vacuum can be reduced to an optimum minimum amount without increasing the over-all cost of the evacuation process.

A still further object of my invention is to provide a method for evacuating an envelope which avoids any risk of fire or explosion and which results in the formation of a vacuum in the envelope that is free from the impurities normally contained in commercially available reducing gases.

Further objects and advantages of the invention will become apparent as the following description of it proceeds and the features of novelty which characterize the invention will be pointed out with particularity in the appended claims. The invention will be better understood by reference to the following detailed description, taken in conjunction with the attached drawings in which:

FIG. 1 is a side elevation, partly in cross section, of one form of apparatus adapted for practicing the evacuating method of my invention to develop a vacuum in an electric discharge device, also depicted.

FIG. 2 is an expanded, perspective view of one of the bonded joint arrangements utilized in forming the electric discharge device depicted in FIG. 1.

Referring now to FIG. 1, there is shown an arrangement of apparatus with which my invention may be practiced. It will be understood that the apparatus depicted in FIG. 1 is merely illustrative of an exemplary form of apparatus that can be used to manufacture by a batch process one or more devices to be evacuated, pursuant to the teachings of my invention. The apparatus illustrated comprises a bell jar 1 sealed to a work bench top 2 by a suitable resilient gasket 3. Within the bell jar 1 there is removably positioned a fiat-top work supporting table 4 on which an electric discharge device 5 rests. An induction heating coil 6, energized from any suitable source through conductors 6a, supplies thermal energy to the metallic surfaces of electric discharge device 5 for a purpose that will be discribed more fully below. It will be appreciated by those skilled in the art that the operating effectiveness of induction coil 6 may be improved by disposing a heat susceptor within bell jar 1 around the exterior surfaces of electric discharge device 5; however, it has been found that in practicing the method of my invention to form a vacuum within envelopes of the type illustrated in FIG. 1, such an additional susceptor is not necessary.

An orifice 7 in the working surface of bench top 2 serves to connect the interior of bell jar 1 with a vacuum pump (not shown), which is connected to the orifice 7 by vacuum-tight tubes 8. In order to prevent back-streaming of oil vapors or other contaminants commonly associated with mechanical vacuum pumps, a molecular sieve trap (not shown) or other suitable means may be disposed in the tubes 8 between the pump and bell jar 1. A reciprocal valve 9 that is adapted to seat on a flexible gasket 10 when in its closed position, as shown, is operated by bellows 1 1 to afford communication between the vacuum pump and the interior of bell jar 1. A second orifice 12 in table top 2 is connected through tube 12a to a pressure gauge 13 that is adapted to sense and indicate the level of pressure within bell jar 1. It will be understood that the apparatus described thus far may be formed of any conventional structures and material. Therefore, substitutions in and modifications of the apparatus discussed may be freely made without departing from the scope of my invention.

For the purpose of fully explaining the pertinent features of my invention, it will be described principally with reference to the manufacture of an electric discharge device 5 of the type that requires a high degree of evacuation and removal of substantially all active gases from its evacuated envelope in order to avoid contamination of the operating electrode element contained therein. By way of example of the order of magnitude of the high vacuum formed in such a device, it is common to require a reduction of pressure in the order of l0 millimeters of mercury within the evacuated envelope. Although a description of the method of my invention will be given in conjunction with the type of electric discharge device depicted as device 5, it must be understood from the outset that the invention is not limited in its application to the manufacture of such electric discharge devices, but rather may be applied to the manufacture of numerous other types of envelopes and evacuated chamber devices which are either high evacuated to form a hard vacuum therein or which may be gas-charged with a rigidly specified predetermined atmosphere. For example, it has been found to be particularly adaptable to the formation of vacuum switches.

The electric discharge device 5 shown in FIG. 1 is composed of a stacked arrangement of elements which are adapted to be bonded together as by brazing or soldering. The electric discharge device 5 comprises an envelope formed by cylindrically shaped ceramic side walls 14, 14' and 14 and cup-like metallic end walls 15 and 16. Although the operating components of electric discharge device 5 are not an essential part of the present invention, it may facilitate an understanding of the invention to describe them generally. The cup-like member 15 includes a planar surface 15a that serves as an anode. A second cup-like member 17 is formed with an annular opening in its planar surface 17a and serves to support a wire grid .18 in operating position between anode 15a and a cathode 19. A final cup-like member 20 is used to support the cathode 19 and its thermionic housing 21 which is conductively mounted in a suitable manner thereon in spaced relation to the grid 18 and anode 15a. A filament heater 22 is disposed in heating conducting relation with the cathode structure 19 and 21 and suitably connected to the electrically conductive cup-like members 16 and 20.

It will be noted that each of the cup-like members 15, 16, 17 and 20 have lip portions designated respectively as 15b, 16a, 17b and 20a. These lip portions provide brazing surfaces as will be discussed more fully below and, in addition, serve as external electrical contact surfaces for the operating components supported thereby. Accordingly, the cup-like members 15, 16, 17 and 20 can advantageously be formed of copper or some other suitably conductive material such as chrome clad iron.

In addition to the foregoing operative elements, a substance capable of evolving gas when heated and sorbing gas when cooled, shown in the form of a disc 23, is supported on member 16 in the envelope formed by walls 14, 14' and 14", and end members 15 and 16. In the preferred embodiment of my invention illustrated in FIG. 1, I prefer to form the disc 23 of titanium hydride or hydrogen charged titanium, because both of these materials are capable of sorbing substantial quantities of hydrogen at rela tively low temperatures and releasing or desorbing hydro gen at elevated temperatures. In applications where relatively large amounts of evolved hydrogen are desired for reducing a pre-existing atmosphere, as explained below, titanium hydride is more preferable because it releases more hydrogen when heated than pure titanium. However, it will be understood that other suitable materials that may be used with my invention include zirconium, cerium, thorium, hafnium, lanthanum, neodymium, praseodymium, vanadium, tantalum, columbium, palladium and the hydrides of any of these materials, since they each also possess the desirable hydrogen sorbing and desorbing characteristics that form a desirable part of my invention. The sorbing characteristics of these materials are described more fully in US. Pat. No. 2,934,392, De Santis et al. which is assigned to the assignee of the present invention.

In order to assure an unobstructed path for the flow of gas from the interior of device 5 to the outside thereof, a corrugated sealing arrangement is provided at the junction formed between the cup-like supporting members 17 and 20 and the ceramic side wall 14 of device 5. These sealing arrangements are identical and are illustrated in an expanded view shown in FIG. 2 of the drawing. Accordingly, the function and structure of only one of these junctions will be described. Referring to FIG. 2, there is shown a portion of ceramic side wall 14 and a portion of ceramic side wall 14' having disposed therebetween the lip 17b of cup-like member 17, a corrugated solder washer 26, planar solder washer 25 and a conductive sealing ring 27. The sealing ring 27 is adapted for surrounding and being bonded to the upper peripheral surface of side wall 14' in any suitable manner, such as by brazing. It will be understood that the normally loose fit between the respective washers 25 and the side walls 14, 14 and 14" would normally provide adequate passageways for gas to flow freely therebetween. However, by providing the corrugated washers 26 at the two junctions noted above, completely free circulation of gas is assured prior to the time that these washers melt to form a metallic bond, as described below.

In practicing the steps of the methods of my invention, to form a hard vacuum in the electric discharge device 5, the device is supported within bell jar 1 in the manner described above with reference to FIG. 1. A relatively inexpensive mechanical pump is connected to the interior of hell jar 1 via tubes 8 and valve 9 which is held open by applying pressure to the lowermost end of diaphragm 11. Simultaneously, electric energy is supplied to induction heating coil 6 through conductors 6a. The mechanical pump rapidly lowers the pressure in bell jar 1 to a pressure in the range between 1 and 100 microns of mercury as indicated by gauge 13. This pressure range is maintained either by continued operation of the vacuum mechanical pump or by closing valve 9 on valve seat 10. As described fully in the above mentioned US. Pat. 2,934,392, De d Santis et al., the temperature range at which the solder joints 25 and 26 will melt to seal the envelope of device 5 can be predetermined by properly selecting the alloys from which the solders are formed. For example, if solder washers 25 and 26 are formed of silver-copper eutectic material, they would be adapted to melt in the range of approximately 780 degrees centigrade. Of course, any suitable solder can be selected for given manufacturing processes without departing from my invention. As the temperature level within the device 5 is raised, the titanium disc 23 desorbs hydrogen as it approaches the 650 degrees centigrade range. Therefore, all of the hydrogen previously sorbed by the titanium is released and it substantially displaces all of the pre-existing atmospheric gases in the device 5 forcing them out of the envelope through the corrugations of washers 26 and subsequently from the bell jar 1 to the vacuum mechanical pump. As the predetermined temperature range is reached (in this case approximately 780 degrees centigrade) the corrugated washers 26 collapsed and fuse with the solder washers 25 to thereby seal the envelope formed by side walls 14, 14 and 14" and end walls 15 and 16. At this point, the induction heating coil 6 is de-energized and the sealed envelope is allowed to cool. As cooling occurs, the titanium disc 23 begins to sorb the remaining hydrogen atmosphere within the envelope of device 5 as it cools into the range between 400 degrees centigrade and 25 degrees Centigrade thereby removing substantially all of the remaining free hydrogen within the envelope and, thus, forming a vacuum that can readily be made as low as 10- millimeters of mercury.

It will be appreciated from the foregoing description of the operational steps employed in practicing the method of my invention that the amount of titanium or other material necessary to form disc 23 may be reduced to a minimum, because the mechanical pumping process applied in combination with the reducing atmosphere created when the hydrogen desorbed by the titanium serves to remove substantially all of the pre-existing gaseous atmosphere from the device 5. This makes it necessary to provide only enough titanium to sorb the small amount of gases remaining from the pre-existing atmosphere and the hydrogen desorbed from the titanium during the heating portion of the method. Moreover, it will be appreciated that since no extraneous source of hydrogen or other active gas is employed in my method to reduce the pre-existing gaseous atmosphere in device 5, there is no opportunity for impurities to be introduced into the device 5 by such gas. On the contrary, only the highly purified hydrogen sorbed by the titanium disc 23 is employed to displace the preexisting gaseous atmosphere from the device 5, thus, substantially all of this gas is readily resorbed into the disc 23 when it is cooled to the predetermined temperature range between 25 and 400 degrees Centigrade.

While it is not essential to the practice of my invention, it has been found that by employing a vacuum gauge such as gauge 13, it is possible to detect exactly when the various steps in the cycle are implemented and, thus, assure precise operation of the method. Specifically, by observing the vacuum gauge 13, the mechanical pumping operation can be terminated and valve 9 closed as soon as the pressure within bell jar 1 is reduced to a desirable predetermined range, such as ten to one hundred microns of mercury. Then, as induction heating coil 6 raises the temperature of the gas containing substance, in this in stance titanium disc 23, causing it to evolve hydrogen, it will be noted that the pressure within ball jar 1 rises by two to three millimeters of mercury, thus, an operator can be assured that proper out-gasing is taking place prior to the time that the solder washers 25 and 26 melt to collapse and seal the envelope of device 5. Even if the mechanical pump is kept in operation, and valve 9 remains open, an increase in pressure will be noted on gauge 13 when the disc 23 desorbs its gases. The pressure indicated on gauge 13 should then remain substantially constant through the remainder of the heating cycle until the Washers 25 and 26 fuse to seal the envelope.

While I have shown and described a specific embodiment of my invention, I do not desire to be limited to the particular form shown and described and I intend by the appended claims to cover all modifications that fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of evacuating an envelope comprising the steps of placing in said envelope a quantity of substance comprising material selected from the group including titanium, zirconium, cerium, thorium, hafnium, lanthanum, neodymium, praseodymium, vanadium, tantalum, columbium, palladium, and the hydrides thereof, heating said substance to desorb gases therefrom, evacuating the envelope to a predetermined low pressure by a mechanical pumping process whereby the atmosphere therein is reduced essentially to gases desorbed from said substance, sealing said envelope while said atmosphere is maintained therein, and cooling said substance for effecting sorption thereby of the gases in said atmosphere.

2. The method of manufacturing an evacuated enevelope for housing an electrical device comprising the steps of mounting said electrical device in an open envelope, including in said envelope a quantity of substance comprising material selected from the group including titanium, zirconium, cerium, thorium, hafnium, lanthanum, neodymium, praseodymium, vanadium, tantalum, columbium, palladium and the hydrides thereof, said quantity of substance being at least sufficient for sorbing a quantity of hydrogen corresponding generally to the volume of said envelope at a predetermined low pressure range, evacuating said envelope to said predetermined low pressure range, heating the substance thereby to desorb gases therefrom which substantially replace the pre-existing gaseous contents of said envelope, sealing said envelope while said substances is desorbing gases and said envelope is evacuated to said predetermined low pressure range, and cooling said substance for effecting sorption of the gases sealed in said envelope, thereby to further exacuate said envelope.

3. The method of manufacturing an envelope comprising the steps of providing an envelope with an opening, providing sealing means for closing said opening, placing in said envelope a predetermined quantity of a substance capable of sorbing a portion of the normal gaseous content of said envelope when said substance is cooled from a relatively high temperature to a lower temperature, elevating the temperature of said substance to said relatively high temperature, evacuating said envelope to a pressure in the range from 1 to microns of mercury, sealing said envelope while the temperature of said substance is elevated and the pressure therein is in the range from 1 to 100 microns of mercury, and permitting said substance to cool for effecting sorption thereby of the gases in said envelope.

4. The method of producing a substantially higher vacuum in an envelope than can be produced by an ordinary mechanical pumping process, which comprises placing in said envelope a small amount of a substance which desorbs an elemental gas when heated to a temperature substantially above normal room temperature and sorbs said gas at temperatures near normal room temperature, removing most of the air from said envelope with an ordinary mechanical pump, heating said substance sufficiently above room temperature to cause it to desorb said elemental gas which displaces the remaining air in said envelope, sealing said envelope, and cooling said sealed envelope and substance to normal room temperature where said substance re-sorbs said gas in said envelope to produce a hard vacuum therein.

References Cited UNITED STATES PATENTS 2,897,036 7/1959 Gale et al 3l625 XR 2,934,392 4/1960 De Santis et al 31625 3,131,983 5/1964 Harries 3l625 H. A. KILBY, JR., Primary Examiner 

